The present invention relates to a developing apparatus, more particularly, to a developing apparatus for an image forming apparatus, such as an electrophotographic copying apparatus, wherein a latent image to be developed is formed on a latent image bearing member, such as a photosensitive member.
Various types of developing methods used with one component developer are known, such as a powder cloud method wherein the cloud is generated by blowing toner powder, a contact method wherein a uniform toner layer formed on a developer carrying member in the form of a web, sheet or roller is directly contacted to a latent image bearing member surface to develop the same, a jumping method wherein a layer of toner particles are not directly contacted to a latent image bearing member surface, but image-wisely transferred to the latent image bearing member surface with the aid of the electric field generated by the latent image, and a so-called magne-dry method wherein a magnetic brush of conductive and magnetic toner particles are contacted to a latent image bearing member surface.
Among such developing methods, the jumping method is known as a peculiar method. In this method, the developing particles are applied on a developer carrying member as a thin layer, whereafter the thin layer of the developer is brought to be opposed to an electrostatic latent image bearing surface with a small gap between the surface of the thin layer of the developer and the latent image bearing surface. Then, the developer particles of the thin layer jump to the latent image bearing surface from the developer carrying member by virtue of the electrostatic force (U.S. Pat. Nos. 2,839,400 and 3,232,190). In this method, the developing operation is performed without a foggy background, since the non-image area of the latent image bearing surface where there is no latent image potential does not attract the developer particles, and in addition, it is not contacted by the developer particles.
However, since the developing operation is based on the jumping action of the toner particles caused by the electric field of the electrostatic image, the developed image is not sharp at an edge of the image, and a line image seems to be thinner than the original. Additionally, the curve of the density of the developed image v.s. the potential of the electrostatic latent image is steep, so that the half-tone reproduction is not good.
A new developing method has been proposed, for example, in U.S. Pat. Nos. 4,356,245, 4,292,387 and 4,395,476, which have been assigned to the assignee of this application, whereby the problems of sharpness and toner reproducibility involved in the conventional jumping developing method have been solved.
Among the three U.S. patents, the first one discloses a method wherein a uniform thin developer layer of one component developer is formed on a developer carrying member and is opposed to a surface of an electrostatic latent image bearing member with a small gap between the surface of the thin developer layer and the surface of the latent image bearing member. The developer is expanded by the electrostatic attracting force by the latent image in the image area to develop the latent image. According to this method, wherein the developer particles do not contact the latent image bearing member in the non-image area, the developed image is sharp and without a foggy background.
In the last two U.S. patents, a thin layer of developer is formed and opposed to the latent image bearing member in the similar manner as above, but an alternating bias voltage as a developing bias voltage is applied between the developer carrying member and the latent image bearing member to produce a reciprocating action of the developer particles, and the clearance or gap between the latent image bearing member surface and the developer carrying member is changed with time, thereby developing the latent image. In the first part of the developing action, the developer particles are transferred to the latent image bearing member in the image area and also in the non-image area so that the half-tone part is developed. With elapse of time, the developer reaches only the image area of the latent image bearing member. According to this method, the resultant image is sharp and without a foggy background, and the toner reproducibility is good.
In these types of developing methods, it is important to form a uniform and thin layer of developer on a developer carrying member. In the case where the developer is a one component magnetic developer, the formation of the thin layer is relatively easy to practice, since the developer is relatively easily controlled by magnetic means. However, in the case where one component non-magnetic developer is used, it is difficult to obtain satisfactory results at all times.
In the conventional developing methods, it is usual that the one component non-magnetic developer particles (toner particles) are supplied to a developer carrying member, such as a developing roller, and electrically charged by a frictional charging member, whereafter they are brought to be opposed to the electrostatic latent image bearing surface. However, if the toner particles supplied to the developing roller are not constant, the toner layer formed on the developer carrying member can be non-uniform. Furthermore, the toner particles can be worn by the friction with the frictional charging member and/or the developing roller so that they are fused and coagulated on the frictional charging member and/or the developing roller surface. This makes the toner layer formation and the electric charge of the toner particles non-uniform. These appear as non-uniform development on the resultant image.
If the surface of the developing roller is roughened with the view to better retention of the toner particle layer thereon, or if a number of small perforations are formed on the surface of the toner particle carrying member with the same view, the toner particles caught by the recesses or the perforations are not easily charged by the charging means, and therefore, do not contribute to the developing operation, so that the resultant image is not good with insufficient image density.
It has further been proposed, in U.S. Pat. No. 4,383,497 that a magnetic brush comprising a developer (toner) and magnetic particles (carrier) is formed on a magnetic roller, and then brought into contact with a developing roller so that only the toner is transferred onto the developing roller and used in the developing action. However, it is difficult to completely block the deposition of the carrier particles onto the developing roller. If the carrier particles deposited on the developing roller are brought to the developing station, the developing bias can leak through the carrier particles between the developing roller and the photosensitive member, and the carrier particles can be transferred to the surface of the photosensitive member, which can damage the surface thereof.